The invention relates to an apparatus for acoustic coupling of ultrasound clamp-on flow measuring heads to high-temperature tube lines (greater than 150° C.).
The determination of the flow of liquid and gaseous media is extremely important in industry and metrology. Ultrasound clamp-on systems, such as are described e.g. in DE 41 14 233 C2, work completely intervention-free. In these systems, the two ultrasound measuring heads are fastened to the tube wall from outside and thereby have no direct contact to the measuring medium and do not affect the flow. The angle between sound propagation direction and flow direction of the measuring medium is determined by the law of refraction, the angle of incidence, and by the acoustic velocity of the measuring head. The transitions between the different materials of the measuring head, the tube wall, and the measuring medium run parallel to one another. The ratio of acoustic velocity and sine of the angle of incidence
      c    i        sin    ⁢                  ⁢          θ      i      corresponding to the law of refraction in all associated media i is the same. The measured flow speed is therefore the proportional to the quotient
      c    i        sin    ⁢                  ⁢          θ      i      
It is established by the material and geometry of the measuring heads and is identified as the sensor constant                kα        
In the aforesaid ultrasound clamp-on flow measurement systems, the measuring heads are coupled directly to the tube wall. The measuring head assumes the temperature of the media and tube due to the direction transmission of heat between tube wall and measuring head. DE 41 24 692 A1 describes a special measuring head that is for use on hot objects and that is characterized by the use of temperature-resistant materials. If the measuring medium and the tube wall have very high temperatures, e.g. greater than 200° C., the measuring head is also correspondingly heated. The severe thermal stress associated with this leads to premature aging in and the inability to function of the measuring heads, e.g. due to depolarization of the piezoceramics that are normally used as transducers. The aforesaid measuring heads for hot objects also do not have the requisite longevity for this stress.
The high-temperature measuring head suggested in U.S. Pat. No. 6,047,602 uses a special waveguide construction for transmitting the ultrasound energy into the measurement tube. This excites shear waves in the waveguide and in the tube wall.
The object of the invention is based on contriving an acoustic coupling that                1. permits a good acoustic coupling between a conventional clamp-on measuring head and the tube wall        2. permits a pronounced difference in temperature between measuring head and tube wall so that the maximum permissible measuring head temperature is not exceeded        3. causes minimal additional measurement errors for the flow speed        